A conventional four port switching valve is disclosed in, for example, Japanese patent application laid-open publications No. 6 (1994)-330712 and No. 7 (1995)-151257. This valve comprises a cylindrical housing having a bore, an inlet port, a first inlet-outlet port, a second inlet-outlet port formed in the cylindrical housing which open to the bore and a spool valve slidably disposed in the bore that can open and close off the section between the inlet port and the first inlet-outlet port and the section between the inlet port and the second inlet-outlet port. The disclosed valve also includes a first ringed groove formed around the spool valve that always opens to the inlet port and can communicate with the first and second inlet-outlet ports. Two drain ports formed in the cylindrical housing so as to open to the bore. A second ringed groove is formed around the spool valve and always opens to one of the drain ports and can communicate with the first inlet-outlet port. A third ringed groove is formed around the spool valve and opens to the other drain port and can communicate with the second inlet-outlet port.
This type of switching valve has two drain ports in the cylindrical housing, which requires many manufacturing processes and a complicated structure. In addition, because two drain ports are formed in the cylindrical housing so that the two drain ports open to the bore, pressurized fluid in the inlet-outlet ports can easily leak through the housing to the drain ports when the pressurized fluid flows from the inlet port to the inlet-outlet ports.